In a radio system such as one for cellular phones, since it is necessary to cover a communication area in a planar manner and since the coverage of a radio wave is limited such that an entire service area cannot be covered by only a single base station, a plurality of base stations (access point: hereafter referred to as “access point AP”) are disposed, so that communication is continuously possible even when a terminal station moves. For example, a system that has a configuration of cells as shown in FIG. 10 is used.
As shown in FIG. 10, a large number of substantially hexagonal cells 39 are disposed in the honeycomb structure in a plane and an access point AP 38 is positioned at the center of each cell 39. A terminal MT 40 existing within each cell 39 is controlled by the access point AP 38, and communication is performed between the terminals MT 40 through the access point AP 38.
In such a structure, in order to continuously provide services such as telephone service even when the terminal MT 40 moves, the cells 39 are disposed in an adjacent to partially overlapping manner. In this case, a different frequency is used for communication in each cell, so that a radio wave is prevented from causing interference in an adjoining cell. When the same frequency is used, the cells are spaced apart from one another by a distance corresponding to several cells so that an interference wave can be sufficiently attenuated. Such a system is referred to as FDMA (Frequency Division Multiple Access) and is used for PDC (Personal Digital Cellular), for example, which is a current digital cellular phone system.
However, in the aforementioned structure using the FDMA method, frequencies practically available in a single cell 39 is only a fraction of the frequencies allotted to the entire system. Therefore, there is a limit to the extent to which the line capacity that can be accommodated within the same cell can be increased.
In view of these facts, it has been proposed to configure cells according to the TDMA (Time Division Multiple Access) method using the same frequency. A method of sharing frequency and time in the TDMA method is explained with reference to FIG. 11. FIG. 11 shows time in the horizontal axis and frequency in the vertical axis. FIG. 11(A) pertains to the FDMA method and FIG. 11(B) shows the usage of frequency in the TDMA method.
As shown in FIG. 11(A), in the FDMA method, separate frequencies f1 to f8, for example, are allotted to the individual users. A user therefore occupies the same frequency on the time axis for communication. Since multiple users exist within a single cell, multiple frequency channels are allotted to each cell.
As shown in FIG. 11 (B), in the TDMA method, a single frequency band is used. The frequency band is divided into narrow slots (time slots), and users communicate by using any of the slots. However, in order to communicate continuously, it is necessary to allot the slots repeatedly to each user. Thus, the slots are allotted to the users at periodic intervals with a repetition period as one cycle shown in the figure.
In the TDMA method, the method of using the time slots in case where there are more than one access point AP, such as access point AP 1 and access point AP 2, for example, is explained with reference to FIG. 12. FIG. 12 shows a system having eight time slots TS 1 to 8.
It is assumed that the two access points AP 1 and AP 2 operate according to a TDMA radio communication method and that the access points use the same number of time slots (repetition period) and the same time slot time (time width of a single time slot). In addition, the time slot time is assumed to be synchronized.
In FIG. 12, between a first access point AP 1 and a terminal (MT, not shown in the figure), a second time slot TS 2 is used for communication. Therefore, seven time slots, namely, TS 1 and TS 3 to TS 8 are vacant. Since interference increases if communication is carried out between a second access point AP 2 and the terminal in the time slot TS 2, they communicate using any one of the seven time slots TS 1 and TS 3 to TS 8. In this way, the same frequency can be shared between different access points AP by dividing it in the time domain.
Moreover, in the Frequency Division Multiple Access (FDMA) method, on account of the limitations of analog circuits such as filters, it is difficult to change the frequency width freely. However, in the TDMA method, there are less circuit limitations because the frequency is divided into slots on the time axis. Therefore, single terminal can use not one but two or three time slots. In this case, the communication capacity can be doubled or tripled, and the bandwidth can be freely controlled for multimedia communication. The TDMA method is thus a communication method also advantageous to packet data communication, for example, wherein the transmission capacity constantly varies.
Power control is an additional technology to realize a radio communication system based on the TDMA method. It is possible that terminals exist from the center of a cell to the edge thereof (cell edge). Because of the characteristics of a radio wave, the attenuation of propagation is low between the access point and a terminal at the center of the cell, while the attenuation is high in the communication with a terminal at the cell edge.
Since the quality that is required for communication is fixed, it is not necessary to have the same transmission level. The emission of unnecessary radio wave can be reduced by lowering the transmission power at the center of the cell and raising the transmission power at the cell edge, for example, thereby maintaining a constant signal power. When the cells are configured in such a method, interference can be reduced not only between adjoining cells for which measures are generally taken against the effects of interference, but also for cells that adjoin the adjoining cells.